1. Field of the Invention
Various embodiments of the present invention relate generally to enhancing the mesoporosity of a zeolite-containing material. More particularly, various embodiments relate to methods for riving a zeolite-containing catalyst.
2. Description of the Related Art
Zeolites and related crystalline molecular sieves are widely used due to their regular microporous structure, strong acidity, and ion-exchange capability. However, their applications are limited by their small pore openings, which are typically narrower than 1 nm. The discovery of MCM-41, with tuneable mesopores of 2 to 10 nm, overcomes some of the limitations associated with zeolites. However, unlike zeolites, MCM-41-type materials are not crystalline, and do not possess strong acidity, high hydrothermal stability, and high ion-exchange capability.
Over the past 10 years, a great deal of effort has been devoted to understanding and improving the structural characteristics of MCM-41. It was found that the properties of Al-MCM-41 could be improved through (1) surface silylation, (2) Al grafting on the pore walls to increase acidity, (3) salt addition during synthesis to facilitate the condensation of aluminosilicate groups, (4) use of organics typically employed in zeolite synthesis to transform partially the MCM-41 wall to zeolite-like structures, (5) preparation of zeolite/MCM-41 composites, (6) substitution of cationic surfactants by tri-block copolymers and Gemini amine surfactants to thicken the walls, and (7) assembly of zeolite nanocrystals into an ordered mesoporous structure. In the latter approach, the first steam-stable hexagonal aluminosilicate (named MSU-S) was prepared using zeolite Y nanoclusters as building blocks. Pentasil zeolite nanoclusters were also used to produce MSU-S(MFI) and MSU-S(BEA).
FIG. 1A is a schematic illustration of a prior art amorphous mesoporous material 100. As shown in FIG. 1A, zeolite nuclei 105a, 105b, 105c were aggregated around surfactant micelles under controlled conditions to form a solid. Thereafter, the aggregated nuclei 105a, 105b, 105c were washed in water and dried and the surfactant was extracted to provide a desired mesopore-sized pore volume 110, forming amorphous mesoporous zeolite nuclei material 100. Each of the zeolite nuclei, for example, 105a, 105b, 105c, is a nanosized crystal. When they are aggregated, the material 100 is polycrystalline because the nuclei material is lacking the long-range regular lattice structure of the crystalline state (i.e., the aggregated nuclei are not fully crystalline or truly crystalline).
Some strategies have managed to improve appreciably the acidic properties of Al-MCM-41 materials. However, due to the lack of long-range crystallinity in these materials, their acidity is not as strong as those exhibited by zeolites. For example, semicrystalline mesoporous materials, such as nanocrystalline aluminosilicate PNAs and Al-MSU-S(MFI), being even more active than conventional Al-MCM-41, showed significantly lower activity than H-ZSM-5 for cumene cracking; the catalyst activity for this reaction has usually been correlated to the Bronsted acid strength of the catalyst.
Previous attempts to prepare mesostructured zeolitic materials have been ineffective, resulting in separate zeolitic and amorphous mesoporous phases. Moreover, some authors have pointed out the difficulty of synthesizing thin-walled mesoporous materials, such as MCM-41, with zeolitic structure, due to the surface tension associated with the high curvature of the mesostructure.